1. Field of the Invention
The present invention relates to a flat display apparatus utilizing an electron beam.
2. Description of the Prior Art
FIG. 1 is a sectional perspective view showing a part of a conventional flat display apparatus as disclosed in, for example, Japanese Patent Laid-Open Publication No. 91-226949 and No. 91-245445 which are patent applications preceding that made by the assignee of the present invention. In FIG. 1, reference numeral 1 designates a heated wire cathode connected to a support member, the cathode emitting electrons when electric conduction is established. Numeral 2 designates a porous cover electrode having an oval cross-section and a multiplicity of holes, said electrode being adapted to cover the upper surface of the wire cathode 1. The multiplicity of small holes provided in the electrode 2 are for the purpose of passing electrons therethrough. By applying an appropriate potential to the electrode 2, electrons are taken out of the wire cathode 1. An electron source 40 is constructed from the wire cathode 1, the porous cover electrodes 2 and the rear electrode 42 which is adapted to secure the porous cover electrodes arranged in parallel to one another and having the same potential as that of the porous cover electrodes 2.
Numeral 4 designates a front glass with the inside surface coated in a dot-like pattern with three kinds of fluorescent materials 5 which emit red, green and blue lights when excited by the electrons drawn out of the electron source 40 and further formed on the fluorescent materials with aluminum film (not shown) for imparting conductivity. The front glass 4 also constitutes a sealed container 413. By applying a voltage of about 10 to 30 kV to the aluminum film, the electrons are accelerated and excite the fluorescent materials 5 so as to emit light. Numeral 6 designates control electrode section which are disposed between the front glass 4 and the wire cathode 1 so as to allow or inhibit passage of the electrons which are taken out by the porous cover electrodes 2 and directed toward the front glass 4. The control electrode section 6 is constructed by a substrate 8 the surface of which is electrically insulated, such as a glass insulating substrate, and which has aperture corresponding to the pixels on the front glass 4, a first control electrode group 9 which is arranged on the surface of the insulating substrate 8 at the side of the electron source such that each control electrode corresponds to each line of the pixels and consists of strip metal electrodes 9a, and a second control electrode group 10 which is arranged on the surface of the insulating substrate 8 at the side of the fluorescent material such that each control electrode corresponds to each row of the pixels and consists of strip metal electrodes 10a.
Each metal electrode of the first and second control electrode groups 9, 10 respectively is composed, for example, of nickel and each comes in the aperture 7. Some portions of the holes are not applied with the nickel film, thereby providing insulation between the first and second control electrode groups.
The first control electrode group 9 is also provided with the insulating grooves or separation zones 44 not applied with the nickel film in the direction intersecting the wire cathode 1. Similarly, the second electrode group 10 is provided with the separation zones 45 in a direction intersecting the first control electrode group 9 or in a direction parallel to the wire cathode 1. These elements are enclosed by the sealed container 43 the interior of which is maintained under vacuum. The respective electrodes are electrically connected externally through the sealed portion provided at the side wall of the container.
Operation of the apparatus will next be explained. The electrons emitted from the heated wire cathode 1 are taken out by the porous cover electrode 2 which is applied with a positive potential of about 5 to 40 V with an average voltage of the wire cathode 1 as the basis (the average voltage is hereinafter assumed to be 0 V). Further, by applying a positive potential of approx. 20 to 100 V to one of the electrodes in the first control electrode group 9 consisting of metal electrodes 9a arranged in a direction orthogonal to the wire cathode 1, the hot electrons are attracted to this electrode and reach the control electrode section 6. By adjusting the elliptic cylindrical configuration of the porous cover electrode 2, the position of the first control electrode group 9 and the voltage applied to the respective metal electrodes 9a, the electron current density at the front of any one of the metal electrodes 9a in the first control electrode group 9 may be made substantially uniform.
Operation of the control electrode section 6 is as follows. As explained above, if only one of the metal electrodes in the first control electrode group 9 has a positive potential applied (or in ON-condition) while the other electrodes have an 0 V or the negative potential applied (or in OFF-condition), the electrons emitted from the wire cathode 1 are attracted only toward the one metal electrode which is in an ON-condition and enter into one line of the apertures 7 provided in the metal electrode 9a. All the electrons which have entered these apertures 7 will not necessarily pass to the side of the front glass 4. More specifically, the electrons pass only through the apertures of the metal electrodes 10a which are in an ON-condition with, for example, a potential of 40 to 100 V applied out of the second control electrode group 10 provided at the side of the front glass 4 and the electrons do not pass through the apertures 7 of the metal electrodes 10a which are in an OFF condition with an 0 V or a negative potential applied.
Accordingly, electrons are allowed to pass through the apertures at the intersection of one metal electrode 9a which is in an ON condition of the first control electrode group 9 and one metal electrode 10a which is in an ON condition of the second control electrode group 10. Passage of electrons through the holes causes the fluorescent material 5 at the pixel corresponding to the aperture 7 to be illuminated so as to provide a display. In other words, by controlling the potential applied to the respective metal electrodes 9a, 10a so that the intersection as above mentioned may coincide with a desired position, desired pictures may be displayed. For example, each one of the metal electrodes 9a in the first control electrode group 9 is sequentially scanned and caused to be ON. Also, the metal electrode 10A in the second control electrode group 10, which corresponds to the position where the light should be emitted, is caused to be ON with the ON-OFF condition of the second control electrode being synchronized with the ON-OFF condition of the first control electrode. That scanning operation mentioned above is repeated in a cycle which is imperceptible to the human eye, 60 frames per second. In this way, pictures-may be displayed.
The respective control electrodes extend into the apertures 7 for the purpose of inhibiting passage of electrons when the respective control electrodes are applied with a small negative potential in the range of 0 V to some 10 volts, such that the electrons which have entered the apertures may be effectively provided with electric fields.
The luminance of each pixel is controlled by the time for which each metal electrode 10a of the second control electrode group 10 is ON. Specifically if it is assumed that the time for which one electrode of the first control electrode group 9 is ON is t.sub.y, and if the luminance of the pixel at a position is intended to be P %, the time t.sub.x for which the metal electrode 10a of the second control electrode group 10 which corresponds to that position is ON is set at P.times.t.sub.y /100.
In such a conventional flat display apparatus, since the metal electrodes which are ON in the first control electrode group 9 is just in the order of one, most of the electrons which have been drawn by the porous cover electrode 2 from the wire cathode 1 will be returned to the side of the electron source due to the negative potential of the metal electrodes which are off, resulting in quite a few electrons which can reach the metal electrodes which are on. This has resulted in such problems as excess consumption of power and insufficient luminance.